Method of making metallic printing screens



June 3, 1947- H. B. FowLER METHOD 0F MAKING METALLIC PRINTING SCREENS 2sheets-sheet` 1 Filed April 3, 1942 imi/1,1,

c .Il l

June 3, 1947. H B FOWLER 2,421,67

METHOD OF MAKING METALLIC PRINTING SCREENS Filed April 3, 1942 2Sheets-Sheet 2 vea/s f1.5@ T t ,fo 55 /f larguer# Patente'd June 3,.1947

UNITED STATES PATENT N osi-ica METHOD OF MAKING BIETALLIC PRINTINGSCREENS Harwood B. Fowler, Wayne, Pa. Application April 3, 1942, SerialNo. 437,543

3 claims. (ol. ion-128.4)

printing screens, avoiding or reducing saw-tooth ing at the printingedges of the designs.

A further purpose is to avoid the necessity of employing extremely nemesh screens for minimizing raw-toothing in tscreen printing, and topermit the use of coarse screens for printing radium dial material andthe like without causing objectionable saw-toothing.

A further purpose is to mask the non-printing" portions of a metallicprinting screen by' a metallic sheet, preferably on the printing face ofthe screen, to secure the metallic sheet and the metallic printingscreen together by a metallic `uniting layer different from the metallicsheet, to etch the design in the metallic sheet, finishing with anetchant which does not substantially attack th'e uniting metal andsubsequently `to remove the uniting metal at the design portions of thescreen, finishing with an etchant selectively attacking the unitingmetal.

A further purpose is to etch a metallic printing screen from theprinting face so that the most perfect-etching of the design will beright at the printing face where etching'starts.

A further purpose is to obtain a sharp etched edge in the design on ametallic sheetunited to a printing screen by placing between themetallic sheet and the screen a metallic layernot substantially attackedby the etchant which finishes the etching of the metallic sheet, themetallic layer separating the metallic sheet from the screen at thepoints of etching.

A further purpose is izo-unite a metallic sheet and a metallic screen byan alloy containing zinc which will be relatively resistant to theetchant which finishes the etching of the metallic sheet, but will beeasily attacked by any usual strong acid.

A further purpose is to make a commercial printing screen of metallicscreen wire,l soldered to a metallic sheet.

A further purpose is to produce a printing screen from metallic screenwire united by a metal to a sheet made of an alloy predominantlycontaining copper.

A further purpose is to. laminate together a metallic screen and ametallic sheet by a metal- I lic bond and to coat the face ofthe screenremote from the metallic sheet with a transparent resist, preferably atransparent lacquer, to delineate `a design through the sheetfandsubsequently to etch out the design thrcughthe uniting metal whileinspecting the face of the screen remote from the metallic sheet throughthe transparent resist for the end point of the etching operation.

A further purpose is to etch a metallic printing screen in several bitesto `avoid break down of the resist.

A further purpose is to regulate the thickness Y of build-up in screenprinting by securing on the printing face ofthe screen a metallic sheetof thickness variable with-respect to the `amount of build-up desired.

A furtherpurpose is to reduce the waste of radium dial material used inprinting.

Further purposes appear in the specification and in the claims.

In the drawings, I have illustrated a few only of the possibleembodiments of the invention, the forms shown being chosen from thestandpoint of convenience in illustration and satisfactory operation.

Figures 1 to 7 'are diagrammatic sections illustrating successive stepsin the construction of the screen of the present invention.

Figure 8 is a diagrammatic section showing electrolytic etching ofthemetallic sheet.

Figures 8a. to 8d are diagrammatic fragments showing successive stagesof etching in Fig-v ure 8.

Figure 8e is a diagrammaticfront elevation of the anode in electrolyticetching.

Figure 9 is an enlarged diagrammatic plan View of a fragment of ascreen, showing how sawtoothing develops. i

Figures 10 and 10a are diagrammatic sections showing alternatetechniques for etching of the uniting metal.

Figures 11 and 12 are diagrammatic sections showing further stepsin`completing the screen.

Figure 13 is a bottom plan view showing the printing face of a nishedscreen.

Figures 14 to 17 are diagrammatic sections showing an alternateprocedure for applying resist to the design face of the screen.

Figures 18 and 19 are diagrammatic sections showing printing withdifferent thicknesses of build-up.`

In the drawings, like numerals refer to like parts throughout. y

In printingmany types of pigments, including ceramic pigments `forlabeling bottles xand jars, and in printing signs on metal and the like,and printing gauges, dials, indications and `i ctions on instruments,apparatus andiequipment, it is frequently necessary to resort to screenprin ing of` the type frequently described as silk screen printing.

The general process consists in blockingoutall portions of thescreenexcept those which are to print the design, and then placing thescreen in close proximity to the workand passing the pigment through thescrenand onto the work pref-l Efforts have therefore been made toemploymetallic screens. These have met with limited success because of thegreat difficulty in securing,

at low cost, smooth and even edges where the' blocked out areas meet thedesign, and the relatively high cost of transferring the design to thescreen in any reasonably dunable fo-rm.

At the present time a great deal of radium and other luminous dialmaterials (herein conven-- tionally referred to as radium dialmaterials) is lapplied to instrument-s, and this work is almost entirelydone by hand because of the impossibility of obtaining the requisitehigh build-up and desired uniformity and precision by the methods ofmachine printing heretofore available. This hand application of radiumdial materials is not only dangerous to the openator, who must carefullyavoid all absorption of the radium by his body, but is also very slowand expensive. Approximately 30 to 40% of the radium dial material usedin the manual process hardens on the brushes, is removed by the cleaningpads, and must be recovered and reworked. One of the important resul-tsachieved by the present invention is the avoidance of the dangerincident to production of radium dials by the manual method and savingof the 30 to 40% radium dial material which has to be reworked in themanual process.

In order to accomplish these and other purposes explained more in detailbelow, the present inventor laminates together a metallic screensuitable for screen printing and a metallic sheet suitable for blockingout the non-printing areas, and unites the screen to the sheet by auniting metal having different etching characteristics from the screenand from the sheet.

. In Figure 1 are shown a metallic printing screen 2li and a metallicsheet 2|. The screen may be of any suitable metal, preferably one of thecorrosion resisting steels (often called corrosion resistant irons) suchas stainless steel. Very satisfactory results are obtained with a screenof stainless steel containing 18 per cent chromium and 8 per centnickel. Corrosion resi-stapt steels or irons containing smaller amountsof chromium with or without nickel maybe used with success, for example,17 per cent of chromium and 2 per cent of nickel, or 16 per centchromium without nickel.

While it is best to employ a screen of stainless steel or the like, itis possible to use a screen predominantly consisting of copper, and madeup, for example, of copper, brass or bronze. This type of screen is lessdesirable than a screen of `corrosion resisting steel, as it is sof-terand less immune to corrosion. I

The mesh size for the screen may be of any usual screen printing mesh,for example, 165 mesh per linear inch (Tyler standard). As will beexplained later, one of the important advantages of the invention isthat much coarser meshes can be employed with satisfaction in thepresent invention than in the prior art.

The metallic sheet 2| preferably predominantly contains copper. Copperor a copper base alloy, such as brass or bronze, may :be used withsuccess.

The metallic sheet and the screen are joined is obtained as well knownin the art.

as 23. In tinning the metallic sheet 2|, it is important to keep thelower surface 24 untinned.

While the screen 20 and the metallic sheet 2| are still very hot andthe-solder layers 22 and 23 are still molten or plastic, the screen andsheet are laminated together by pressure between suitable dies 25 and26, las shown in Figure 3. A pressure of a few hundred pounds, asobtained in a hand press, is sufficient for this purpose. 'I'he dies 25and 26 will desirably be heated, though the temperature need not be ashigh as the melting point of solder in every case. Where the dies areheated above the melting point of vthe solder, they will be Iallowed tocool sufliciently to permit the solder to harden before the pressure isreleased. The result as produced in Figure 3 is soldering together ofthe adjoining surfaces of the screen and the metallic sheet by a layerof solder 21 which extends between the screen and the metallic sheet andfills the interstices of the screen. In accomplishing this soldering, itis best to have the screen above the metallic sheet, so that solder willnot necessarily fully fill the meshes of the screen for the entirethickness of the screen, although lt will extend over the entire area.

The thicknesses of the solder layers 22, 23 and 2'|y are exaggerated inthe drawing.

After suitable cooling of the laminated screen and sheet, and invertingof the laminated structure, the front or non-printing surface of thescreen is coated with a resist 28, preferablya transparent resist asshown in Figure 4. The best transparent resist is a transparent lacquersuch as nitrocellulose lacquer, ester gum lacquer or glycerol-phthalicacid lacquer. Any suitable transparent lacquer, which can later bedissolved away, may be employed.

The printing face 24 of the metallic sheetis then coated with aphotosensitive layer 29 (Figure 5) as Well known in the art. Thephotosensitive layer 29 may actually be several layers applied until thedesired thickness of photosensitive layer Any suit.- ablephoto-engravers sensitizing solution of the cold enamel type may be usedin making the photosensitive layer 29, as for example, dichromatedgelatine. A highly satisfactory sensitizing solution of the cold enameltype is sold onv the market as Hess special re-etching enamel.

After the photosensitive layer 29 has been properly dried (underprotection from light strong enough to cause exposure), it isphotographically exposed to the copy which contains the design to betransferred to the screen. The most convenient way to do this is bycontact printing technique as shown in Figure 6. The copy 30 suitablyreversed (because it is being photographed against the printing surfacerather than the so-called front surface of the screen) is placed againstthe photosensitive layer 29, in a contact printing frame, if desired,and a strong light is applied for a time long enough to complete theexposure. Areas 3| where the design appears on the coply will protectthe photosensitive layer immediately beneath them from exposure, butother areas 32, where the design is absent will permit transmission oflight to expose the portions of the photosensitive layer immediatelybeneath. The copy 30 may be either a photos negative or a photo-positivetransparency on film or glass.

After exposure the photosensitive layer is developed by any developerwell known in the art by the exposure into a resist 35 as shown inFigure 7.

An important feature of the invention is the etching in at` least twosteps, the rst of which etches through the metallic sheet and the secondof which removes the uniting metal at corresponding points.

For etching through the metallic sheet, it is best to use anelectrolytic etching process, though this is of course not essential. Itis found, however, that electrolytic etching gives very rapid, sharp andstraight-walled etching of the metallic sheet.

In Figure 8, the laminated screen and sheet form the anode 36 in anelectroplating bath 31 contained within a vessel 38` A cathode 39suitably of copper is connected inthe circuit with a direct currentsource 40.

the screen aids greatly in obtaining a straight and smooth etched edge4| of the copper.

It has been found that the etchant first produces a gradually iilleteddepression 42 near the edge of the resist as shown in Figure 8a. As theetching becomes deeper, this curve becomes sharper as shown at 43 inFigure 8b. Further etching results in a configuration as shown at 44 inFigure 8c. If the solder layer 21 is not present, it has been found thatthe tendency is` to leave a curved projection 45 not removed by etching.If the etching is prolonged without any solder layer being present, theresultis to obtain undercutting from the back and edge of the metallicsheet before the projection 45 is fully etched away.

The solder layer 21, however, not being substantially attacked by theetchant for the copper, tends to prevent undercutting while the copperetchant is attacking and removing the projection 45 until thecomparatively straight; etched wall 4| is obtained as shown in Figures 8and 8d.

Thus it is evident that the square character of the etched edge of themetallic sheet is improved by the presence of the layer of uniting metal21 not attacked by the etchant.

The layer of uniting metal 21 performs a-further very important functionduringy the copper etch. If the metallic sheet 2| and the screen 20 werein direct contact with one another during the copper etch, without anyuniting layer being The plating current l will preferably be about 8amperes and the volt-` present, the etchant would tend to crawl alongthe wires 46 of the screen beneath the copper and to follow the squaresof the screen mesh in etching, producing a saw-toothed outline 41(Figure 9), instead of following the line 48 of the design. Thissaw-toothed `outline 41 attained in the prior art, is very undesirableas it is carried over into the outline of the iinal pigment build-up inprinting. i

It has been found that the presence of the solder layer 21, preventingall physical contact between the wires 49 and the vmetallic sheet 2|during the etching lof the metallic sheet, eliminates the saw-toothingentirely and produces a smooth etched wall (which of course may becurved to suit the design) following the line 48.

By virtue of the fact that the uniting metal layer 21 is not copper andis resistant to copper` etchants, the uniting layer of soldernot onlyassists by uniting the metallic sheet and the metallic screeniirmlytogether, but also aids -greatly in obtaining a square etched Wall ofthe copper sheet in depth and a in length.

In order to attempt .to avoid saw-toothing in smooth etched wall screenswhere the metallic screen and the'metal` lic sheet have been in directcontact during the etch, it has been necessary to employ 'very finescreen meshes. This does not in fact prevent saw-teething, but it makesthe saw-toothing so line that it is not very noticeable `in the buildupon the final printing. For this reason, it has been the prior practiceof the inventor to employ screen mesh sizes of about 165 Tyler standardmesh per linear inch, and coarser screens could not be used withoutexhibiting very obvious saw-toothing.

This is a distinct disadvantage in printing certain pigments which aredesirably of large particle size. For example, in printing radium dialmaterials, where the production of eliicient luminous coatingsnecessitates the employment ofparticles of relatively large size,.it hasnot been possible to obtain satisfactory printing through the linescreens needed in the prior practice to avoid saW-toothing, andaccordingly, manual application of the radium dial material has beenemployed with corresponding great risk to workers who are likely to comeinto physical contact with the radium dial material.

By the present invention, the screen mesh, no matter how coarse it maybe, exerts no undesirable effect upon the squareness or smoothness ofthe etched wall produced in the metallic sheet, since the solderprevents physical contact between the metallic sheet `and the screenduring the etching of the sheet. Thus, it has been possible inaccordance with the present invention to produce etched screens of meshor even coarser without any saw-teething taking place at the printingface. With such coarser screens, satisfactory printing of radium dialmaterials is possible not only for comparatively crude commercial uses,but more especially to meet the very exacting requirements of militaryaircraft instruments and those required in mechanized land warfare.

One ofthe important features of the invention is that the etching isstarted at the printing face.

The more the etching advances into` the depth of the sheet, the morepronounced will its tendency be to deviate from the contour of thedesign.

Thus, if lin spite of the precautions taken in' accordance with thepresent invention, some tendency-to saw-toothing is evident, it islikely to develop only in the portions of the laminated screen and sheetwhich are relatively far from the printing face 24. Thus a certainamount of inaccuracy incident to high production methods can betolerated without destroying the utility of the invention by marring theaccuracy and Vsmoothness of the outline at the printing face.

Such slight inaccuracy will merely affect the etching in the region 44which is relatively far removed from the printing face.

While the procedure above indicated for making ythe copper etch can beused satisfactorily, it is comparatively slow and ineiective. Much morerapid etching can be obtained if a solution of ferrie chloride isinitially employed in the electrolytic bath to etch through the copperuntil the uniting layer is first exposed. Ferrie chloride etches muchmore rapidly than chromlc acid and can be used quite efficiently up toabout the stage shown in Figure 8c where the uniting layer has rst beenuncovered. It is not safe to use ferric chloride for etching beyond thestage of uncovering the uniting layer (except as noted below) as it willattack the uniting layer and will not discriminate between the copperand the uniting layer. Therefore the etching solution should be changedafter reaching the stage of Figure 8c. lThe laminated screen and .sheetshould be washed, dried and then'further electrolytically etched inchromic acid solution to remove the remaining copper, such as the curvedprojections 45.

Both ferrie chlorideV and chromlc acid have a tendency to break down theresist 35 if etching is carried on continuously from beginning to end.It seems to be a softening eiect due to the Water present rather than adirect chemical attack. It is found that this diiculty can Ibe avoidedby etching in a plurality of short bites, and drying the laminatedscreen and sheet between bites. Each drying is preferably preceded by awash in water. Each drying tends to harden the resist lso that it canwithstand the action of the etchant during the next succeeding bite. Forbest results the copper should be removed, using several (preferablyfour or five) bites with ferric chloride solution and then several(preferably four'or live) bites with chromic acid solution, washing anddrying Ibetween each bite; by short bites, I mean bites of about ve, tenor fifteen minutes.-

It is very difficult to regulate the electrolytic etching and obtainuniform etching over the surface of the design, particularly near theend of the etch whenA most of the copper to be removed has been etchedout. I have discovered that regulation of the electrolytic etching canbe greatly improved if a current bleed is used. In Figure 8e, I show theprinting face of the cornposite screen in the electrolytic solution witha current bleeding electrode 50, suitably of sheet iron or of tin plate,surrounding the laminated l surrounding it in the same` plane, and isapproximately as far from the cathode as is the composite screen.

As the end of the copper etch approaches, in the condition between thatshown in Figure 8c and 8d, the effective current density in the copperto be etched tends to increase to a tremendously high level.

The bleeding electrode maintains a constant relatively large area forcurrent distribution, so that the electrolytic etching conditions changeless markedly toward the end of the copper etch than would be the caseif the bleeding electrode were not present. The bleeding electrodetherefore `operates as a balance or regulating device to maintainrelatively constant etching conditions toward the end of the copperetch, when the effective current density from the standpoint of thecopper being etched would otherwise increase to an abnormally highvalue, with a tendency to accelerate the copper etching up to an uncorrtrollable rate.

After etching through the copper sheet 2|, the laminated screen andsheet is removed from the etchant, washed with water and dried.

Having removed the copper throughout thel area of the design, it isnecessary that the uniting metal layer 21 be removed also atcorresponding points. This is accomplished by a second etching as shownin Figure 10` or 10a. The etchant for the second etching is one whichwill preferentially attack the solder rather than the metallic sheet orthe screen. For this purpose, it is preferred to employ one of the usualstrong mineral acids forming a soluble lead salt, preferablyhydrochloric (muriatic) acid in concentrated form. Instead ofhydrochloric acid, nitric acid is suitable, preferably in concentrationsbetween 25 and 40%. Nitric acid is not particularly desirable as anetchant, where the screen predominantly consists of copper, and for acopper screen, hydrochloric acid is much more satisfactory. However,Where a corrosion resistant steel screen is used, nitric acid is fairlyVsatisfactory as a solder etchant, although it is less desirable thanhydrochloric acid. Any other suitable solder etchant maybeused.

It will be evidentV that lthe solder etchant should preferentiallyattack the solder ratherl than the metallic sheet or the screen. Asalready explained, the metallic sheet and the screen will preferably beof different metallic compositions, but this is not necessarily so asthey could beof the same metallic composition, but of differentcomposition from the solder. The solder of course protects the metallicscreen from attack during the etching of the sheet, no matter what thecomposition of the metallic screen may be.

While ordinary solder, such as 50 per cent leadtin solder, is verysatisfactory for making the solder layer 2l, it has been found thatthere is a little difliculty in removing ordinary lead-tin solder byetching with an etchant which does not seriously attack the copperduring the time of etching.

In order to facilitate removal of the solder layer, it is thereforepreferred to use as a solder an alloy containing zinc. Pure zinc issatisfactory for the layer 2l, but very much better soldering resultsare obtained by an alloy containing about 37% lead, 37% tin and 25%zinc. Even much smaller percentages of zinc incorporated in lead orlead-tin alloys, greatly improve the etching, and advantage will. beobtained by adding any amount of zinc in excess of 1% to the metal ofthelayer 21.

It has been round that the removal ofthe solder layer 21 is a delicateoperation, as too prolonged etching will tend to cause undercutting andweaken the joint between the copper sheet and the screen at areas whichare blocked out. It is therefore desirable to use special etchingtechnique by which the non-printing side of' the screen is subjected toobservation during the latter part of the etching.

In Figure 10, the laminated screen and sheet are placed with theprinting surface uppermost on a suitably elevated glass plate 5|desirably surroundedby a confining wall 52. The etchant solution isshown at 53, where it can attack the solder over the area covered by thedesign, and remove it from between the interstices of the screen atpoints 54. Over-etching will cause the etchant to travel laterally inthe space between the copper sheet and the screen.

To prevent over-etching, the non-printing face of the screen has beenmade subject to observation by using a transparent resist 28, suitably atransparent lacquer. By the mirror 55,.it is then possible for theoperator to view the bottom surface of the screen 20 in Figure 10through the transparent resist 28. Usually the etching time should beabout 20 minutes. As soon as the design is clearly delineated in themirror and before the spreading of the design is possible, etching isstopped and the composite screen removed from the etchant, washed anddried.

The presence of the layer of lacquer 28 tends itself to preventlcreeping since the etchant can not pass around under the screen duringetching,

but only attack the solder from the sides in order to spread laterally.Y

Various other techniques may beused to accomplish the etching and` onesuch valternate technique is shown in Figure a. In this form, thecomposite screen is etched with its printing face down and the operatorwatches through the transparent lacquer 28 until the design is clearlydelineated on the upper surface, at which time he removes the compositescreen from the etchant before the etchant can undercut. The techniqueof Figure 10 is more efiicient as the etchant will best etch downwardly.

Where very rapid etching is desired, possibly at the expense of acertain amount of accuracy,

it is possible to accelerate the solder etch by employing aselectrolytic etch withferric chloride at 'the beginning of the solderetch, and then,after taking a short bite with ferric chloride, washing,drying, and finishing the etching in the manner already described usinghydrochloric acid, nitric acid, or some other acid and a plain (notelectrolytic) etch. fFerric chloride solution attacks stainless steeland other corrosion resistant steel to some extent, but it attacks thesolder much more rapidly and can be used to accelerate the solder etchwhile doing comparatively little damage to the screen and to the edgesof the copper sheet. The ferric chloride bite at the beginning of thesolder etch should preferably not be more than about ten minutesV long.

After washing and drying the fully etched composite screen, the lacqueris removed by a suitable solvent, for example acetone, in the case ofnitrocellulose lacquer, so that the interstices of the screen 2U areexposed at the design areas as shown at 56 in Figure 11,

In case any difficulty is encountered in clearing the openings of themesh at points 56, the comproduct, as shown in Figure 12, consists ofthe screen 20 united tothe metallic sheet 2| `at the blanked out areasby the solder layer 21, and completely exposed in the design areas 56.If desired` the entire composite screen may be lightly plated, forexample, copper plated. The view shown in Figure 13 of the printing faceillustrates the fact that the blanked out area=is` completely covered bythe metallic sheet 2|, the screen being visible only at the printingface in'the design areas 56.`

It should be noted that the outline 50 of the design, as shown in Figure13, is substantially free from saw-toothing, and is etched smoothly andsquarely down, as shown in Figures 8d, 10 and 10a.

In some cases it may be desirable to apply the acid resist to the areasof the metallic sheet which are to be blanked out without the necessityof using a photosensitive layer on the metallic sheet. This can beaccomplished, as shown in Figures 14 to 17, by printing the resistthrough a silk or Wire screen. Figure 14 illustrates thecopy 30 havingdesign areas 3| and areas 32 to be blanked out. The copy 30 is a reverseprint of the desired design in that it has light transmitting areas at32 where it is desired to have the final design located-and lightobscuring areas at 3| which are to be nally blanked out. l

The copy 30 is used to make a contactprint on a photographic layer 58superimposed on a suitable silk or wire screen 51, as well known in theart. After the contact print has been exposed and developed, the screen51 will be open and free from obstructions at areas 59 (Figure 15) whichit is desired to have blanked out in the final product, and will beblanked out at areas 60, Where it is desired to have the,` design in thefinal product. Y The screen 51 islsuperi-mposed upon the compositescreen of the invention whose metallic sheet is uppermost against thescreen 51. A suitable resist, such as gelatine, is screen printed in Ythe areas 59 to be blanked out, while it is excluded from the areasbelow the masking layer 6D (Figure 16).

The resultant product after tanning, or other-- Wise suitably hardeningthe resist, where required, will be the composite screen of theinvention as shown in Figure 17, coated with a layer of resist 35 andhaving the design areas 33eXposed for etching. The product of Figure 17,afteraipplying a suitable transparent lacquer coatingto the exposed faceof the screen, is ready for etching according to the proceduresindicated in Figures 8 and 10 or 10a and treatment according to thesubsequent steps of the invention In the prior art, great diiiiculty hasbeen encountered in obtaining different thicknesses of build-up in thefinished printing, and efforts have been made to control the build-up bychanging the consistency of the pigment,` changing the completion ofprinting and lifting ofthe com` posite screen and sheet away fromcontact with` the surface being printed. The surface on which theprinting has taken place is visible at 6l and the raised printing orbuild-up is shown as B2. In this case the metallic sheet 2i' isrelatively thick and therefore the screen 20 has been relatively farabove the surface 6 I, on which build-up is taking place, during theprinting. Correspondingly there is a high build-up at 62.

In Figure 19 the metallic sheet 2 I2 in the composite screen is muchthinner and therefore the build-up 62 on the surface 6| is not nearly sothick.

The solder layer 21 is practically invariable in thickness in all casesand, in fact, its thickness from the edge of the screen adjoining themetallic sheet is almost infinitesimal.. Due to the necessities ofdrawing technique, the thickness of the solder layer 21 has beenexaggerated in Figures 18 and 19.

The thickness of the metallic sheet 2| may vary between 0.0015 and 0.015inch. For normal sign printing, the metallic sheet will usually be about0.002 inch thick. For printing of radium dial material, it has beenfound to be desirable, and in fact, almost necessary, to use a thicknessconsiderably greater and best results are obtained when the metallicsheet is about 0.005 to 0.008 inch thick. Using a metallic sheet of thisthickness and with the other conditions as indicated herein, it ispossible to obtain very satisfactory radium printing on the faces anddials of various instruments of military importance, with adequatethickness of build-up and adequately large particle size to obtainefcientluminosity, and with accurately defined and sharp edges asrequired to obtain the precision needed in printing on such instrumentsof importance to national defense. f

The radium printing pigment employed with the composite screen of thepresent invention may be any suitable prior art radium printing pigment,but preferably of much higher viscosity than used heretofore for handapplication. In such hand application, the radium printing pigment hasnormally been thinned to the consistency of ordinary paint. For thescreen printing of the present invention, the radium printing pigmentwill normally be used at a viscosity about that of S. A. E. 120 oil.This normally requires no thinner. It is also preferable to employ aslow-drying lacquer, such as a four to seven hour lacquer, in the radiumprinting pigment used according to the present invention.

One of the advantages of the present invention is that requisiteaccuracy and control of build-up can be obtained by very rapidoperations, but nevertheless the printing face of the screen isextremely resistant to wear and capable of making numerous impressionseven on surfaces which are not of perfect regularity.

The user, by stocking various composite screens having differentmetallic sheet thicknesses, can be prepared to vary the build-up atWill, without changing the .operating technique used in printing, ordeparting from the usual squeegee procedure.

In view of my invention and disclosure variations and modifications tomeet individual whim 4 or particular need will doubtless become evidentclaim as new and desire to secure by Letters Patent is:

1. The process of producing a printing screen having a design, whichcomprises laminating together a metallic screen and a metallic sheet bya uniting layer of a solder of different metallic composition coveringthe exposed face of the screen with a. transparent resist,y blocking outthe areas around the design by a resist applied to the exposed face ofthe metallic sheet, etching through the metallic sheet, finishing by anetchant preferentially attacking the metallic sheet and etching throughthe uniting layer, finishing by an etchant preferentially attacking theuniting layer, saidtransparent resist functioning to permit observationofthe etching of the uniting layer'by inspection therethrough.

2. The process of producing a printing screen having a design, Whichcomprises laminating together a metallic screen and a metallic sheet bysolder, coating the exposed face of the metallic screen by a transparentresist, coating the areas of the printing face of the metallic sheetother than the design areas with a resist, etching the design throughthe metallic sheet, nishing by an etchant preferentially attacking themetallic sheet, and etching the design through the solder, finishing bylan etchant preferentially attacking the solder, said transparent resistfunctioning to permit observation of the etching of the solder byinspection of the opposite side of the screen therethrough.

3. The process of producing a printing screen having a design; whichcomprises soldering together in side by side relation a metallic screenof corrosion resistant steel and a metallic sheet predominantlycontaining copper, coating the exposed face of the screen with atransparent resist, protecting the areas around vthe design on theexposed surface of the sheet which is to be the printing face by aresist photographically applied, etching the design through the copperof the sheet, finishing byv an etchant preferentially attacking thecopper, etching the designthrough the solder, nishing by an etchantpreferentially attacking the solder, said transparent resist functioningto permit observation of the face of the screen remote from the etchingtherethrough, and then removing the transparent resist.

.H ARWOOD B. FOWLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,376,365 Wertheirner Apr. 26,1921 909,831 Strecker Jan. 12, 1909 2,213,237 Brennan Sept. 3, 19402,177,877 Pfaffmann Oct. 31, 1939 2,058,365 Stark Oct, 20, 19362,233,546 Meulendyke Mar. 4, 1941 2,282,203 Norris May 5, 1942 2,267,787Ciavola Dec, 30, 1941 2,338,091 Brennan Jan. 4, 1944 1,908,487 PowersMay 9, 1930 2,290,554 Hack July 2, 1942 2,288,020 Noland June 30, 1942 fFOREIGN PATENTS Number Country Date 454,162 Great Britain Sept. 26, 1936

